Field of Invention
The present invention relates to chemical detection utilizing Raman spectroscopy, including but not limited to military use and, in particular, to apparatus and methods for efficiently interrogating a sample (e.g. solids, liquids, solutions, mixtures) at different laser wavelengths to allow acquisition of multiple Raman spectra from the sample to assist in separating Raman information about the sample from background signal, including fluorescence excited by the laser.
Related Art
The value of Raman spectroscopy for an ever-increasing variety of applications is widely discussed in the literature. One example is non-destructive detection of one or more chemical constituents of potentially dangerous or harmful objects or substances, such as explosives, chemical warfare agents, and toxic industrial chemicals. Non-destructive interrogation of such things, at a reasonable stand-off distance, can be invaluable. Raman spectroscopy has applications in other areas, including but not limited to, pharmaceutical, petrochemical, and other manufacturing/inspection processes.
As will be discussed further below, Raman spectroscopy has a technical hurdle for many of its proposed applications. Raman scattering from an interrogation energy (e.g. laser beam) normally consists of a minute (e.g. on the order of 1:109) fraction of the total scattering. Non-Raman signal dominates. Many times auto-fluorescence from the target caused by the interrogating laser is returned and obscures or confounds the Raman spectra. A variety of attempts to overcome this hurdle have been proposed in the literature. This includes interrogating the target at more than one laser wavelength. Comparison of the returned total signal at different laser interrogation wavelengths has been found to help identify and separate Raman spectra from non-Raman spectra. Non-Raman spectra can include Rayleigh scattering, auto-fluorescence, and background noise,
However, the inventors have identified that there is room for improvement in the manner in which different interrogation laser wavelengths can be created. They have also identified beneficial techniques for controlling spectra acquisition from the multiple wavelength interrogations. As can be appreciated by' those skilled in this technical field, a variety of factors can be relevant to the design and operation of a Raman spectrometer, Several have been discussed above. But additional factors can complicate such design. Some laser sources are tunable but not over a large bandwidth, This limits its performance regarding Raman spectroscopy. It is possible to add somewhat complex components including multiple lasers to get different laser wavelengths, as exemplified in US Pat. No. 7,245,369, incorporated by reference herein, but this can be expensive and impractical. Therefore, the factors regarding how a laser source can be tuned over effective and efficient bandwidth by relatively noncomplex techniques are not necessarily straightforward because of competing and sometimes antagonistic factors involved. Economy also can come into play. Many of the systems proposed in the state of the art do not meet these tests. Some require separate components, such as a laser cavity and some sort of device outside the cavity to control switching or change of laser wavelengths. Others utilize relatively expensive components. Still others need laboratory-type controlled settings. And, further, some systems require very accurate calibration or settings to work adequately. This would seem to make sense because of the small amount of detectible energy involved and the issues of signal to noise discussed earlier.
In particular, for non-laser diode laser generation, a laser gain medium in a laser cavity is needed. Line narrowing components or elements are included to produce a line-narrowed laser for effective laser interrogation of a target or sample for acquiring Raman spectra. However, satisfactory techniques for tuning a laser over a satisfactory gain bandwidth have not been identified in the art.
Therefore, the inventors have identified a need in the art for improvement regarding apparatus and methods for acquiring Raman spectra at different interrogation laser wavelengths, including use with Raman spectroscopy for chemical detection.